1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an array substrate for a fringe field switching (FFS) mode LCD device capable of having a multi-domain structure.
2. Discussion of the Related Art
A related art liquid crystal display (LCD) device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite alignment direction as a result of their thin and long shapes. The alignment direction of the liquid crystal molecules can be controlled by application of an electric field across the liquid crystal molecules. As the intensity or direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Since incident light is refracted based on the orientation of the liquid crystal molecules due to the optical anisotropy of the liquid crystal molecules, images can be displayed by controlling light transmissivity.
Since the LCD device including a thin film transistor (TFT) as a switching element, referred to as an active matrix LCD (AM-LCD) device, has excellent characteristics such as high resolution and display of moving images, the AM-LCD device has been widely used.
The AM-LCD device includes an array substrate, a color filter substrate and a liquid crystal layer interposed therebetween. The array substrate may include a pixel electrode and TFT, and the color filter substrate may include a color filter layer and a common electrode. The AM-LCD device is driven by an electric field between the pixel electrode and the common electrode resulting in excellent properties of transmittance and aperture ratio. However, since the AM-LCD device uses a vertical electric field, the AM-LCD device has a bad viewing angle.
An in-plane switching (IPS) mode LCD device may be used to resolve the above-mentioned limitations.
FIG. 1 is a schematic cross-sectional view of the related art IPS mode LCD device. In FIG. 1, the related art IPS mode LCD device includes a color filter substrate 9 as an upper substrate, an array substrate 10 as a lower substrate facing the color filter substrate, and a liquid crystal layer 11 interposed therebetween. Both common and pixel electrodes 17 and 30 for driving the liquid crystal layer 11 are formed on the array substrate 10. The liquid crystal layer 11 is driven by a horizontal electric field “L” induced between the common and pixel electrodes 17 and 30.
FIGS. 2A and 2B are cross-sectional views showing turned on/off conditions of an IPS mode LCD device according to the related art. As shown in FIG. 2A, when the voltage is applied to the IPS mode LCD device, liquid crystal molecules 11a above the common electrode 17 and the pixel electrode 30 are unchanged. But, liquid crystal molecules 11b between the common electrode 17 and the pixel electrode 30 are horizontally arranged due to the horizontal electric field “L”. Since the liquid crystal molecules are arranged by the horizontal electric field, the IPS mode LCD device has a characteristic of a wide viewing angle. FIG. 2B shows a condition when the voltage is not applied to the IPS mode LCD device. Because an electric field is not generated between the common and pixel electrodes 17 and 30, the arrangement of liquid crystal molecules 11 is not changed.
Unfortunately, the IPS mode LCD device has disadvantages in an aperture ratio and transmissivity. To resolve these limitations, an FFS mode LCD device, where the liquid crystal molecules are driven by a fringe field, is introduced.
FIG. 3 is a plan view showing one pixel region the related art FFS mode LCD device. In FIG. 3, on an array substrate 41 for the FFS mode LCD device, a gate line 43 is formed, and a data line 51 is formed to cross the gate line 43 such that a pixel region “P” is defined. In each pixel region “P”, a thin film transistor Tr as a switching element is formed. The TFT “Tr” is connected to the gate and data lines 43 and 51 and includes a gate electrode 45, a gate insulating layer (not shown), a semiconductor layer (not shown), a source electrode 55 and a drain electrode 58. A pixel electrode 60 is formed in the pixel region “P”. The pixel electrode 60 is connected to the drain electrode 58 of the TFT “Tr” through a drain contact hole 59 and includes a plurality of openings “op” having a bar shape. The pixel electrode 60 has a plate shape. In addition, a common electrode 75 is formed in the pixel region “P” to overlap the pixel electrode 60. The common electrode 75 covers an entire surface of a display region of the array substrate 41. A portion of the common electrode 75 corresponding to one pixel region “P” is marked as a dotted-line.
In the FFS mode LCD device having the above structure, voltages are applied to the pixel and common electrodes 60 and 75 to generate a fringe field therebetween. Unfortunately, the related art FFS mode LCD device has a mono-domain structure such that an azimuth angle is changed depending on a position of the user. For example, a color shift problem is generated at an azimuth angle such as 0, 90, 180 and 270 degrees, such that an image quality is degraded.